In a circuit board of a motherboard or module on which electronic components such as a semiconductor component are mounted, sometimes a malfunction occurs. The malfunction may be generated by inter-device mutual interference caused by electromagnetic wave due to a circuit operation. The malfunction may also be generated by a noise that propagates on a ground surface or a power supply surface. The malfunction of the circuit board, which is caused by the mutual interference or the noise, emerges particularly in the case that the circuit board is downsized, namely, a mounting layout area of the electronic component is reduced.
In order to prevent the malfunction of the circuit board, which is caused by the mutual interference or the noise, for example, a distance between the mounted electronic components may be increased, or a filter circuit in which an electronic component is used is added to the circuit board. However, the increase of the distance between the electronic components or the addition of the filter circuit obstructs the downsizing of the circuit board.
On the other hand, there is adopted a method for covering the semiconductor component subject to the electromagnetic wave or a whole electronic circuit on the circuit board subject to the electromagnetic wave using a metallic casing. The semiconductor component or the electronic circuit is physically protected by the method, and the harmful electromagnetic wave from the outside is cut off by the method.
An EBG (Electromagnetic Band Gap) structure provided in a dielectric circuit board is proposed, because an effect to reduce a circuit area is enhanced compared with the filter circuit in which the electronic component such as semiconductor device is used.